I have an iPhone 4S running iOS6 (although I was having this issue pre-iOS6). My iPhone sends iMessages, so long as they are text, fine, delivers read receipts, everything fine when I'm on WiFi. It used to send photos on all the WiFi connections I typically use (home, campus, favorite coffee shop, etc.) but about a week prior to iOS 6 it decided it would no longer send photos via iMessage. When I want to send a photo I have to turn off my wireless, and send the photo on data. Then I turn wireless right back on and resume sending texts via iMessage. I have no issues with other WiFi functions - web browsing, etc. Is unaffected.
SMS and MMS are on. My iMessaging is directly linked to my phone number, not my Apple ID, if that makes a difference?
Hoping someone can help with this, especially since it appears its not my wireless signal thats the problem as my iPhone has decided to do this over multiple different wireless signals it used to not have an issue with.